Hmm... Needless to say that these figures don't inspire much confidence in simulator platforms of various brands.
On the other hand, my THD figures obtained from MicroCap were reasonable in line with actual measurements on the 'blameless' amp, Bob's HEC amp and the PGP amp.
I also like to add that, at least to me, a figure of 26ppm doesn't sound overly optimistic.

>Its curious that the results get polarized into these two different groups of results.
Sure. But who's to blame? The user or the tool?

The deal with simulators and accurate THD sims is that you need "good models" without them you dont get accurate simulated results. I am not sure of the quality of the parts used in the last simulation exercise but I dont think its fair to draw the conclusion that simulators are unreliable for THD simulations.

The deal with simulators and accurate THD sims is that you need "good models" without them you dont get accurate simulated results. I am not sure of the quality of the parts used in the last simulation exercise but I dont think its fair to draw the conclusion that simulators are unreliable for THD simulations.

"After following some tips from the LTSpice mailing list, I see no practical limitation from the FFT accuracy (in LTSpice).
With compression off, alternative solver on, running
.tran 0 12m 0 .1u gives .fourier results down to 3E-6 (110dB dynamic range). The graphical FFT, selecting 65536 points and Hann window, gives a "noise floor" of -200dB. And if you join Bernhard's Church of the Merciless Noise Killing, he will most likely give you the setup for getting down to -300dB.
But these are only numbers in a theoretical game, as the active devices' models are not anywhere near this accuracy.

Regards,
Peter Jacobi"

That's okay, but now look at this one:

"Regarding the accuracy of the FFT (needed for THD analysis) within PSPICE I found the following:http://www.<b>orcadpcb.com</b>/kb_ar...20019.asp?bc=F
an excerpt:
Simulation accuracy is limited to about 3-3.5 digits (60-70dBc). The accuracy can be improved by tightening tolerances and taking the above considerations to extremes, but not by very much. This may not be considered sufficient for some applications, but keep in mind that even the best simulation models are probably not accurate enough to provide better
[...]
Regards
Charles "

3.5 digits correspond with an accuracy of ~300ppm. If this is true, it explains a lot! Orcad